1. Field of the Invention
The present invention relates to an optical branching unit, which is usable in the field of optical communication, and outputs a light beam input to an input end to a plurality of output end.
2. Description of the Related Art
A PLC (Planer Light Wave Circuit) component is known as an optical waveguide circuit component. As an example of the PLC component, an example has been reported, in which a pattern of an optical waveguide circuit is formed on a silicon substrate, and a partially independent core pattern is provided on the substrate provided with the optical waveguide circuit (Jpn. Pat. Appln. KOKAI Publication No. 11-271545).
However, in the proposal disclosed in the above Publication No. 11-271545, there is a problem peculiar to the structure of an optical branching that is a PLC component. Namely, the number (area) of waveguides is different in a light input side and a light output side, and an etching rate in dry etching for example is largely different on a substrate by a loading effect. As a result, after the etching, a line width of a curved waveguide in the input side is formed thinner than a designed value, and a loss in the curved waveguide is increased in a longer wavelength side of a wavelength in use. As a result, the uniformity in a wavelength of insertion loss of an optical branching unit becomes bad.
Further, after the etching, a clad layer is provided in all areas of a substrate including a core portion formed by etching, and the clad layer is protruded like a convex in a core portion. Thus, when a lid glass is fixed with an adhesive, a stress is concentrated on the core portion, and PDL (Polarization Dependent Loss) is degraded.